Sequence of Structural and Magnetic Phase Transitions in (NO)Mn6(NO3)13

New nitrosonium manganese(II) nitrate, (NO)Mn6(NO3)13, has been synthesized and structurally characterized. In the temperature range of 45-298 K, the crystal is hexagonal (centrosymmetric sp. gr. P63/m). Mn2+ ions are assembled into tubes along axis c with both NO3- filling and coating. The nitrosonium cation is located in the framework cavity and is disordered by a 3-fold axis. At the temperature TS1 = 190 K, a structural phase transition related to the libration of the intertube NO3 group and a small variation of Mn polyhedron is observed. Moreover, the anomalies in physical properties of (NO)Mn6(NO3)13 allow suggesting that ordering of NO+ units occurs at low temperatures. The antiferromagnetic ordering in this compound is preceded by the formation of a short-range correlation regime at about 25 K and takes place in two steps at TN1 = 12.0 K and TN2 = 8.4 K.

A combination of organic and inorganic cations in the synthesis of transition metal nitrates: preparation and characterization of canted rectangular Ising antiferromagnet (PyH)CsCo2(NO3)6

Pyridinium cesium cobalt nitrate, (PyH)CsCo2(NO3)6, obtained from a nitric acid solution crystallizes in the orthorhombic space group Pnma with unit cell parameters a = 8.6905(14) Å, b = 11.9599(18) Å, c = 18.386(3) Å, V = 1911.0(5) Å3, and Z = 4. It consists of [Co(NO3)3]- layers, in which each Co2+ ion is connected with four monodentate bridging NO3-groups and one bidentate terminal NO3-group, forming a corrugated rectangular net. Magnetization and specific heat measurements show that (PyH)CsCo2(NO3)6 undergoes a long-range canted antiferromagnetic ordering in two steps at TC1 = 5.0 K and TC2 = 2.6 K. The temperature dependence of the magnetic susceptibility and the field dependence of the magnetization measured for (PyH)CsCo2(NO3)6 show that it is an Ising antiferromagnet. In support of these observations, our DFT + U + SOC calculations show that the Co2+ ions of (PyH)CsCo2(NO3)6 have an easy-axis magnetic anisotropy with preferred spin orientation along the b-axis. To a first approximation, the spin lattice of (PyH)CsCo2(NO3)6 is a weakly alternating Ising antiferromagnetic chain (J1/J2 ∼ 0.85), and these chains interact weakly (J3/J2 ∼ 0.07) to form a rectangular Ising antiferromagnetic lattice. In agreement with the prediction for a rectangular Ising antiferromagnet by Onsager, (PyH)CsCo2(NO3)6 undergoes a long-range antiferromagnetic ordering.

A cascade of magnetic phase transitions and a 1/3-magnetization plateau in selenite-selenate Co3(SeO3)(SeO4)(OH)2 with kagomé-like Co2+ ion layer arrangements: the importance of identifying a correct spin lattice

We prepared a new compound, Co₃(SeO₃)(SeO₄)(OH)₂, having layers in a kagomé-like arrangement of Co²⁺ (spin S = 3/2) ions. This phase crystallizes in the orthorhombic space group Pnma (62) with unit cell parameters a = 11.225(9) Å, b = 6.466(7) Å and c = 11.530(20) Å. Its layers, parallel to the ab-plane, are made up of Co1O₅ square pyramids and Co2O₆ and Co3O₆ octahedra. As the temperature is lowered, Co₃(SeO₃)(SeO₄)(OH)₂ undergoes three successive magnetic transitions at 27.5, 19.4 and 8.1 K, and the magnetization of Co₃(SeO₃)(SeO₄)(OH)₂ measured at 2.4 K exhibits a 1/3-magnetization plateau between 7.8 and 19.9 T. The H-T magnetic phase diagram constructed for Co₃(SeO₃)(SeO₄)(OH)₂ from ac and dc magnetic susceptibility, specific heat and magnetization measurements contains three magnetic phases I, II and III. Phase I is antiferromagnetic, while phases II and III are ferrimagnetic and responsible for the 1/3-magnetization plateau. To interpret these complex magnetic properties, we identified the correct spin lattice for Co₃(SeO₃)(SeO₄)(OH)₂ by evaluating its intralayer and interlayer spin exchanges based on spin-polarized DFT+U calculations.

Anhydrous copper tellurite disulfate Cu3TeO3(SO4)2 featuring the coexistence of spin singlets and a long-range antiferromagnetic order

Anhydrous copper tellurite sulfate, Cu₃TeO₃(SO₄)₂, has been synthesized via vapor transport reactions in sealed silica glass ampoules. In measurements of magnetization M, magnetic susceptibility χ, specific heat C_p and X-band electron spin resonance, a long-range antiferromagnetic order at T_N = 13 K and an H-T magnetic phase diagram have been established. One-third of Cu²⁺ ions were found to form magnetically silent dimers. A peak in dielectric permittivity ε, which accompanies the Néel order, allows considering Cu₃TeO₃(SO₄)₂ as a magnetoelectric multiferroic material of the second type. Density functional theory calculations provided estimations of leading exchange interaction parameters.

Magnetic Phase Diagram of van der Waals Antiferromagnet TbTe3

Terbium tritelluride, TbTe₃, orders antiferromagnetically in three steps at T_N1 = 6.7 K, T_N2 = 5.7 K, and T_N3 = 5.4 K, preceded by a correlation hump in magnetic susceptibility at T* ~8 K. Combining thermodynamic, i.e., specific heat C_p and magnetization M, and transport, i.e., resistance R, measurements we established the boundaries of two commensurate and one charge density wave modulated phases in a magnetic field oriented along principal crystallographic axes. Based on these measurements, the magnetic phase diagrams of TbTe₃ at H‖a, H‖b and H‖c were constructed.

Ising-like Magnetism in Quasi-Two-Dimensional Co(NO3)2·2H2O

The appearance of electrically neutral water molecules in the structure of cobalt dinitrate dihydrate, Co(NO₃)₂⋅2H₂O, drastically changes its magnetic properties as compared to its waterless counterpart, Co(NO₃)₂. The title compound shows Ising-like behavior reflected in its thermodynamic properties. It experiences long-range antiferromagnetic order at T_N = 20.5 K and metamagnetic transition at µ₀H_C = 0.76 T. First-principles calculations produce the values of leading exchange interactions J1 ~ 10 K and J2 ~ 0.5 K and single-ion anisotropy D ~ 1 K which allows us to consider Co(NO₃)₂⋅2H₂O as a quasi-two-dimensional magnetic system.

Nonstoichiometric Ellenbergerite-Type Phosphates: Hydrothermal Synthesis, Crystal Chemistry, and Magnetic Behavior

We synthesized single crystals of Na_0.55Ni₆(OH)₃(H_0.61PO₄)₄ (I) and polycrystals of (Na, Ni)_0.64Ni_5.68(OH)₃(H_0.67PO₄)₄ (II) with ellenbergerite-like structures using the hydrothermal method. The phases crystallize in the hexagonal space group P6₃mc with the following unit cell parameters: a = 12.5342(1) Å, c = 4.9470(1) Å, and V = 673.08(2) ų for I; a = 12.4708(2) Å, c = 4.9435(2) Å, and V = 665.82(2) ų for II; and Z = 2. Their crystal structures are based on a 3D framework built from NiO₆ octahedra and PO₄ tetrahedra. The difference between I and II lies in the way the structural channels are filled along the [001] direction. These channels accommodate segments of Na- and (Na, Ni)-centered chains of face-sharing octahedra in the structures I and II, respectively. The magnetic susceptibility χ and the specific heat C_p evidence pronounced low-dimensional magnetic behavior at elevated temperatures and the formation of the weakly ferromagnetic long-range order at T_N^I = 61 K and T_N^II = 63 K. Analysis of the χ(T) data within both chain and dimer spin models allows the estimation of the leading exchange interaction parameters in the compounds under study.

Sawtooth chains self-assembled from clusters of MnO6 octahedra within the silicate framework of K3Mn4Si10O24.33(H2O,OH)3/V,B

A disordered mineralogically probable silicate hydrate K3Mn4Si10O24.33(H2O,OH)3/V,B, obtained hydrothermally, demonstrates low-dimensional magnetic behavior.

[Study of Multiple Enzymatic Incorporation of Modified Nucleotides of Purine and Pyrimidine Nature in the Growing DNA Chain]

The substrate properties of nitrogen-base modified derivatives of purine and pyrimidine deoxynucleoside triphosphates during their simultaneous pairwise insertion into the growing DNA strand have been studied. Modified nucleotides were introduced using real-time PCR and the primer extension reaction; in one reaction, derivatives with both different and similar functional substituents were used. Genomic bacterial DNA, specially constructed synthetic DNA fragments, and SELEX libraries were used as templates. The reactions were performed using DNA polymerases with no 3'-5' correcting exonuclease activity: Taq, Vent (exo-), DeepVent (exo-), and KOD XL. It was shown that the substrate efficiency is affected by both the size of the substituent group and the chemical nature of deoxynucleoside triphosphate. The effectiveness varies significantly depending on the polymerase used. The most effective of the studied substrates are pyrimidine deoxynucleoside triphosphates in combination with Vent (exo-) DNA polymerase. DNAs modified by pairs of dissimilar nucleotides (dU + dC, dU + dA, dC + dA) with similar and different functional substituents were obtained.

Liver and COVID-19: possible mechanisms of damage

The global epidemic of a new coronavirus infection caused by SARS-CoV-2 is a major threat to human health. In the clinical picture, along with acute respiratory distress syndrome, liver lesions are also noted. The following mechanisms are currently being considered: direct damaging effects of SARS-CoV-2, immuno-mediated inflammation, hypoxia, drug exposure, and reactivation of pre-existing liver disease. We studied 150 patients with COVID-pneumonia who are under inpatient treatment at the University Clinical Hospital No. 1 of Sechenov First Moscow State Medical University. Of these, the presence of SARS-CoV-2 RNA was confirmed by polymerase chain reaction in 84 (56.0%) patients. In 55 (36.7%) patients, an increase in serum aminotransferases was registered, mainly alanine aminotransferase max. up to 572 U/L and aspartate aminotransferase up to a max. of 232 U/L. The long-term consequences are unknown and require monitoring of these patients.

[Risk factors for the early development of septic shock in patients with severe COVID-19]

AIM

In a retrospective study, we evaluated factors associated with the early development of septic shock in patients with severe COVID-19.

MATERIALS AND METHODS

We collected medical records of the intensive care unit patients submitted by the local COVID-19 hospitals across Russia to the Federal Center for the Critical Care at the Sechenov First Moscow State Medical University (Sechenov University). Septic shock in crticially ill patients requiring mechanical ventilation was defined as a need in vasopressors to maintain blood pressure.

RESULTS

We studied 1078 patients with severe COVID-19 who were admitted to the intensive care units for respiratory support. There were 611 males and 467 females. The mean age was 61.013.7 years. Five hundred twenty five medical records (48.7%) were received from the Moscow hospitals, 159 (14.7%) from the Moscow region, and 394 (36.5%) from the hospitals located in 58 regions of the Russian Federation. In 613 (56.9%) patients, diagnosis of SARS-CoV-2 infection was confirmed by PCR, and in the other cases it was established on the basis of the clinical picture and the results of the chest CT scan. Septic shock developed in 214 (19.9%) of 1078 patients. In the logistic regression model, the risk of septic shock in patients older than 50 years was higher than in patients of a younger age (OR 2.34; 95% CI 1.533.67; p0.0001). In patients with more severe SARS-CoV-2 infection, there was an increase in the prevalence of cardiovascular diseases, including coronary heart disease and atrial fibrillation, type 2 diabetes and malignant tumors. The risk of septic shock in patients with three or more concomitant diseases was higher than in patients without any concomitant chronic diseases (OR 1.76; 95% CI 1.762.70).

CONCLUSION

The risk of septic shock in patients with acute respiratory distress syndrome induced by SARS-CoV-2 is higher in patients older than 50 years with concomitant diseases, although a severe course of the disease is also possible in younger patients without any concomitant disorders.

Structure, conductivity and magnetism of orthorhombic and fluorite polymorphs in MoO3-Ln2O3 (Ln = Gd, Dy, Ho) systems

Phase-pure orthorhombic compositions at a Ln/Mo ratio ∼ 5.2-5.7 (Ln = Gd, Dy, Ho) have been obtained for the first time by prolonged (40-160 h) heat treatment of mechanically activated 5Ln2O3 + 2MoO3 (Ln = Gd, Dy, Ho) oxide mixtures at 1200 °C. Although the starting Ln : Mo ratio was 5 : 1 (Ln10Mo2O21 (Ln = Dy, Ho)), it changed slightly in the final product due to the volatility of molybdenum oxide at 1200 °C (40-160 h) (ICP-MS analysis). Brief high-temperature firing (1600 °C, 3 h) of 5Ln2O3 + 2MoO3 (Ln = Gd, Dy, Ho) oxide mixtures leads to the formation of phase-pure fluorites with compositions close to Ln10Mo2O21 (Ln = Gd, Dy, Ho). Gd10Mo2O21 molybdate seems to undergo an order-disorder (orthorhombic-fluorite) phase transition in the range of 1200-1600 °C. For the first time, using the neutron diffraction method, it was shown that low-temperature phases with a Ln/Mo ratio ∼ 5.2-5.7 (Ln = Gd, Dy, Ho) have an orthorhombic structure rather than a tetragonal structure. Proton contribution to the total conductivity of Ln10Mo2O21 (Ln = Gd, Dy, Ho) fluorites and gadolinium and dysprosium orthorhombic phases in a wet atmosphere was observed for the first time. In both orthorhombic and fluorite phases, the total conductivity in wet air decreases with decreasing lanthanide ionic radii. In a wide temperature range, the compounds under study exhibit paramagnetic behaviour. However, the orthorhombic phases of Dy and Ho compounds reach the antiferromagnetic state at 2.4 K and 2.6 K, respectively.

Multifunctional Compound Combining Conductivity and Single-Molecule Magnetism in the Same Temperature Range

We report the first highly conducting single-molecule magnet, (BEDO)₄[ReF₆]·6H₂O [1; BEDO = bis(ethylenedioxo)tetrathiafulvalene], whose conductivity and single-molecule magnetism coexist in the same temperature range. The compound was synthesized by BEDO electrocrystallization in the presence of (Ph₄P)₂[ReF₆]·2H₂O and characterized by crystallography and measurements of the conductivity and alternating-current magnetic susceptibility.

Spin-singlet Quantum Ground State in Zigzag Spin Ladder Cu(CF3 COO)2

The copper salt of trifluoroacetic acid, Cu(CF₃ COO)₂ , offers a new platform to investigate the quantum ground states of low-dimensional magnets. In practice, it realizes the ideal case of a solid hosting essentially isolated magnetic monolayers. These entities are constituted by well-separated two-leg half-integer spin ladders organized in a zigzag fashion. The ladders are comprised of dimeric units of edge-sharing tetragonal pyramids coupled through carbon ions. The spin-gap state in this compound was revealed by static and dynamic magnetic measurements. No indications of long range magnetic ordering down to liquid helium temperature were obtained in specific heat measurements. First principles calculations allow estimation of the main exchange interaction parameters, J_⊥ =176 K and J_∥ =12 K, consistent with the weakly interacting dimers model.

Magnetically frustrated synthetic end member Mn2(PO4)OH in the triplite-triploidite family

The manganese end member of triplite-triploidite series of compounds, Mn₂(PO₄)OH, is synthesized by a hydrothermal method. Its crystal structure is refined in the space group P2₁/c with a = 12.411(1) Å, b = 13.323(1) Å, c = 10.014(1) Å, β = 108.16(1), V = 1573.3 ų, Z = 8, and R = 0.0375. Evidenced in measurements of magnetization M and specific heat C_p, Mn₂(PO₄)OH reaches a long range antiferromagnetic order at T_N = 4.6 K. As opposed to both triplite Mn₂(PO₄)F and triploidite-type Co₂(PO₄)F, the title compound is magnetically frustrated being characterized by the ratio of Curie-Weiss temperature Θ to Néel temperature T_N of about 20. The large value of frustration strength |Θ|/T_N stems from the twisted saw tooth chain geometry of corner sharing triangles of Mn polyhedra, which may be isolated within tubular fragments of a triploidite crystal structure.

Crystal structure and spin-trimer magnetism of Rb2.3(H2O)0.8Mn3[B4P6O24(O,OH)2]

The novel borophosphate Rb_2.3(H₂O)_0.8Mn₃[B₄P₆O₂₄(O,OH)₂] was prepared under hydrothermal conditions at 553 K. Its crystal structure was determined using single-crystal X-ray diffraction data obtained from a non-merohedral twin and refined against F² to R = 0.057. The compound crystallizes in the orthorhombic space group Pbcn, with unit-cell parameters a = 20.076(2) Å, b = 9.151(1) Å, c = 12.257(1) Å, V = 2251.8(2) ų, and Z = 4. The title compound is the first example of a borophosphate with manganese ions adopting both octahedral and tetrahedral coordinations. Its unique crystal structure is formed by borophosphate slabs and chains of Mn²⁺-centered polyhedra sharing edges and vertices. These 2D and 1D fragments interconnect into a framework with open channels that accommodate Rb⁺ cations and water molecules. Topological relationships between borophosphates built from three-membered rings of two borate and one phosphate tetrahedra sharing oxygen vertices, amended by additional PO₄ and HPO₄ tetrahedra, are discussed. The temperature dependence of the magnetic susceptibility of Rb_2.3(H₂O)_0.8Mn₃[B₄P₆O₂₄(O,OH)₂] reveals predominant antiferromagnetic exchange interactions and the high-temperature effective magnetic moment corresponding to the high-spin S = 5/2 state of Mn²⁺ ions. At 12.5 K, a magnetic transition is evidenced by ac-susceptibility and specific heat measurements. A spin-trimer model with the leading exchange interaction J ∼ 3.2 K is derived from density-functional band-structure calculations and accounts for all experimental observations.

Crystal Structure, Defects, Magnetic and Dielectric Properties of the Layered Bi3n+1Ti7Fe3n-3O9n+11 Perovskite-Anatase Intergrowths

The Bi_3n+1Ti₇Fe_3n-3O_9n+11 materials are built of (001)_p plane-parallel perovskite blocks with a thickness of n (Ti,Fe)O₆ octahedra, separated by periodic translational interfaces. The interfaces are based on anatase-like chains of edge-sharing (Ti,Fe)O₆ octahedra. Together with the octahedra of the perovskite blocks, they create S-shaped tunnels stabilized by lone pair Bi³⁺ cations. In this work, the structure of the n = 4-6 Bi_3n+1Ti₇Fe_3n-3O_9n+11 homologues is analyzed in detail using advanced transmission electron microscopy, powder X-ray diffraction, and Mössbauer spectroscopy. The connectivity of the anatase-like chains to the perovskite blocks results in a 3a_p periodicity along the interfaces, so that they can be located either on top of each other or with shifts of ±a_p along [100]_p. The ordered arrangement of the interfaces gives rise to orthorhombic Immm and monoclinic A2/m polymorphs with the unit cell parameters a = 3a_p, b = b_p, c = 2(n + 1)c_p and a = 3a_p, b = b_p, c = 2(n + 1)c_p - a_p, respectively. While the n = 3 compound is orthorhombic, the monoclinic modification is more favorable in higher homologues. The Bi_3n+1Ti₇Fe_3n-3O_9n+11 structures demonstrate intricate patterns of atomic displacements in the perovskite blocks, which are supported by the stereochemical activity of the Bi³⁺ cations. These patterns are coupled to the cationic coordination of the oxygen atoms in the (Ti,Fe)O₂ layers at the border of the perovskite blocks. The coupling is strong in the n = 3, 4 homologues, but gradually reduces with the increasing thickness of the perovskite blocks, so that, in the n = 6 compound, the dominant mode of atomic displacements is aligned along the interface planes. The displacements in the adjacent perovskite blocks tend to order antiparallel, resulting in an overall antipolar structure. The Bi_3n+1Ti₇Fe_3n-3O_9n+11 materials demonstrate an unusual diversity of structure defects. The n = 4-6 homologues are robust antiferromagnets below T_N = 135, 220, and 295 K, respectively. They show a high dielectric constant that weakly increases with temperature and is relatively insensitive to the Ti/Fe ratio.

Structure-Property Relationships in α-, β'-, and γ-Modifications of Mn3(PO4)2

The manganese orthophosphate, Mn₃(PO₄)₂, is characterized by the rich variety of polymorphous modifications, α-, β'-, and γ-phases, crystallized in monoclinic P2₁/c (P2₁/n) space group type with unit cell volume ratios of 2:6:1. The crystal structures of these phases are constituted by three-dimensional framework of corner- and edge-sharing [MnO₅] and [MnO₆] polyhedra strengthened by [PO₄] tetrahedra. All compounds experience long-range antiferromagnetic order at Neel temperature T_N = 21.9 K (α-phase), 12.3 K (β'-phase), and 13.3 K (γ-phase). Additionally, second magnetic phase transition takes place at T* = 10.3 K in β'-phase. The magnetization curves of α- and β'-modifications evidence spin-floplike features at B = 1.9 and 3.7 T, while the γ-Mn₃(PO₄)₂ stands out for an extended one-third magnetization plateau stabilized in the range of magnetic field B = 7.5-23.5 T. The first-principles calculations define the main paths of superexchange interaction between Mn spins in these polymorphs. The spin model for α-phase is found to be characterized by collection of uniform and alternating chains, which are coupled in all three directions. The strongest magnetic exchange interaction in γ-phase emphasizes the trimer units, which make chains that are in turn weakly coupled to each other. The spin model of β'-phase turns out to be more complex compared to α- or γ-phase. It shows complex chain structures involving exchange interactions between Mn2 (Mn2', Mn2″) and Mn3 (Mn3', Mn3″). These chains interact through exchanges involving Mn1 (Mn1', Mn1″) spins.

Synthesis, structure and magnetic ordering of the mullite-type Bi2Fe(4-x)CrxO9 solid solutions with a frustrated pentagonal Cairo lattice

Highly homogeneous mullite-type solid solutions Bi2Fe(4-x)CrxO9 (x = 0.5, 1, 1.2) were synthesized using a soft chemistry technique followed by a solid-state reaction in Ar. The crystal structure of Bi2Fe3CrO9 was investigated using X-ray and neutron powder diffraction, transmission electron microscopy and (57)Fe Mössbauer spectroscopy (S.G. Pbam, a = 7.95579(9) Å, b = 8.39145(9) Å, c = 5.98242(7) Å, RF(X-ray) = 0.022, RF(neutron) = 0.057). The ab planes in the structure are tessellated with distorted pentagonal loops built up by three tetrahedrally coordinated Fe sites and two octahedrally coordinated Fe/Cr sites, linked together in the ab plane by corner-sharing forming a pentagonal Cairo lattice. Magnetic susceptibility measurements and powder neutron diffraction show that the compounds order antiferromagnetically (AFM) with the Néel temperatures decreasing upon increasing the Cr content from TN ∼ 250 K for x = 0 to TN ∼ 155 K for x = 1.2. The magnetic structure of Bi2Fe3CrO9 at T = 30 K is characterized by a propagation vector k = (1/2,1/2,1/2). The tetrahedrally coordinated Fe cations form singlet pairs within dimers of corner-sharing tetrahedra, but spins on the neighboring dimers are nearly orthogonal. The octahedrally coordinated (Fe,Cr) cations form antiferromagnetic up-up-down-down chains along c, while the spin arrangement in the ab plane is nearly orthogonal between nearest neighbors and collinear between second neighbors. The resulting magnetic structure is remarkably different from the one in pure Bi2Fe4O9 and features several types of spin correlations even on crystallographically equivalent exchange that may be caused by the simultaneous presence of Fe and Cr on the octahedral site.

An open framework crystal structure and physical properties of RbCuAl(PO4)2

Single crystals of the RbCuAl(PO₄)₂ compound with an open 3D framework structure were obtained by a hydrothermal route. The compound orders antiferromagnetically at T_N = 10.5 K and exhibits spontaneous magnetization at lower temperature.

A novel cobalt sodium phosphate hydroxide with the ellenbergerite topology: crystal structure and physical properties

The novel phosphate hydroxide Na_2−xCo₆(OH)₃[HPO₄][H_x/3PO₄]₃with the mineral ellenbergerite topology and alkaline cations in the framework channels shows strong antiferromagnetic interaction and magnetic transition atT_N= 44 K.

Anisotropy of magnetic properties of Fe(1+y)Te

The magnetic properties of Fe(1+y)Te single crystals (y ≃ 0.1 ÷ 0.18) were studied at temperatures 4.2 ÷ 300 K. At an ambient pressure, with decreasing temperature a drastic drop in χ(T) was confirmed at T ≃ 60 ÷ 65 K, which appears to be closely related to the antiferromagnetic (AFM) ordering. It is found that the magnitudes of the anisotropy of magnetic susceptibility Δχ in the AFM phase are close in the studied samples, whereas the sign of the anisotropy apparently depends on the small variations of the excess iron y in Fe(1+y)Te samples. The performed DFT calculations of the electronic structure and magnetic properties for the stoichiometric FeTe compound indicate the presence of frustrated AFM ground states. There are very close energies and magnetic moments for the double stripe configurations, with the AFM axes oriented either on the basal plane or along the [0 0 1] direction. Presumably, both these configurations can be realized in Fe(1+y)Te single crystals, depending on the variations of the excess iron. This can provide different signs of magnetic anisotropy in the AFM phase, presently observed in the Fe(1+y)Te samples. For these types of AFM configuration, the calculations for the FeTe values of Δχ are consistent with our experimental data.

Crystal structure, physical properties, and electronic and magnetic structure of the spin S = 5/2 zigzag chain compound Bi2Fe(SeO3)2OCl3

We report the synthesis and characterization of the new bismuth iron selenite oxochloride Bi2Fe(SeO3)2OCl3. The main feature of its crystal structure is the presence of a reasonably isolated set of spin S = 5/2 zigzag chains of corner-sharing FeO6 octahedra decorated with BiO4Cl3, BiO3Cl3, and SeO3 groups. When the temperature is lowered, the magnetization passes through a broad maximum at Tmax ≈ 130 K, which indicates the formation of a magnetic short-range correlation regime. The same behavior is demonstrated by the integral electron spin resonance intensity. The absorption is characterized by the isotropic effective factor g ≈ 2 typical for high-spin Fe(3+) ions. The broadening of ESR absorption lines at low temperatures with the critical exponent β = 7/4 is consistent with the divergence of the temperature-dependent correlation length expected for the quasi-one-dimensional antiferromagnetic spin chain upon approaching the long-range ordering transition from above. At TN = 13 K, Bi2Fe(SeO3)2OCl3 exhibits a transition into an antiferromagnetically ordered state, evidenced in the magnetization, specific heat, and Mössbauer spectra. At T < TN, the (57)Fe Mössbauer spectra reveal a low saturated value of the hyperfine field Hhf ≈ 44 T, which indicates a quantum spin reduction of spin-only magnetic moment ΔS/S ≈ 20%. The determination of exchange interaction parameters using first-principles calculations validates the quasi-one-dimensional nature of magnetism in this compound.

Magnetic properties of superconducting FeSe in the normal state

A detailed magnetization study for the novel FeSe superconductor is carried out to investigate the behavior of the intrinsic magnetic susceptibility χ in the normal state with temperature and under hydrostatic pressure. The temperature dependences of χ and its anisotropy Δχ = χ([parallel]) - χ([perpendicular]) are measured for FeSe single crystals in the temperature range 4.2-300 K, and a substantial growth of susceptibility with temperature is revealed. The observed anisotropy Δχ is very large and comparable to the averaged susceptibility at low temperatures. For a polycrystalline sample of FeSe, the significant pressure effect on χ is determined to be essentially dependent on temperature. Ab initio calculations of the pressure-dependent electronic structure and magnetic susceptibility indicate that FeSe is close to magnetic instability, with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume and especially on the height Z of chalcogen species from the Fe plane. The change of Z under pressure determines a large positive pressure effect on χ, which is observed at low temperatures. It is shown that the literature experimental data on the strong and nonmonotonic pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with the calculated behavior of the density of electronic states at the Fermi level.

Quasiparticle dynamics and phonon softening in FeSe superconductors

Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements (ΔR/R) provides unprecedented information on Fe-based superconductors. The amplitude of the fast component in ΔR/R clearly gives a competing scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 and 230 K. The former manifests a structure phase transition as well as the associated phonon softening. The latter suggests a previously overlooked phase transition or crossover in FeSe. The electron-phonon coupling constant λ is found to be 0.16, identical to the value of theoretical calculations. Such a small λ demonstrates an unconventional origin of superconductivity in FeSe.

Pressure effects on the magnetic susceptibility of FeTe(x)(x approximately 1.0)

The magnetic susceptibility χ of FeTe(x) compounds (x approximately 1.0) was studied under hydrostatic pressure up to 2 kbar at fixed temperatures of 55, 78 and 300 K. Measurements were taken both for polycrystalline and single crystalline samples. At ambient pressure, with decreasing temperature a drastic drop in χ(T) was confirmed at T approximately 70 K, which appears to be closely related to antiferromagnetic ordering. The obtained results have revealed a puzzling growth of susceptibility under pressure, and this effect is enhanced by lowering the temperature. To shed light on the pressure effects in the magnetic properties of FeTe, ab initio calculations of its volume dependent band structure and the exchange enhanced paramagnetic susceptibility were performed within the local spin density approximation.